Method for producing bokon-carbon fibers

ABSTRACT

1. A METHOD FOR CONTINUOUSLY DEPOSITING NODE-FREE AMORPHOUR BORON TO UNIFORM THICKNESSES GREATER THAN 0.6 MIL ON A MOVING RESISTIVELY HEATED CARBON WIRE AS IT IS DRAWN THROUGH A SERIES OF REACTORS COMPRISING THE STEPS OF: MAINTAINING THE CARBON WIRE IN THE FIRST REACTOR AT A TEMPERATURE OF 1600*-2100*C.; EXPOSING THE CARBON WIRE WHILE AT SAID TEMPERATURE IN AT LEAST ONE REACTOR TO A GASEOUS STREAM CONSISTING ESSENTIALLY OF A CARBON-CONTAINING GAS ADMIXED WITH A DILUENT GAS TO EFFECT DEPOSITION OF PYROLYTIC GRAPHITE ON THE CARBON WIRE; MAINTAINING THE GRAPHITE-COATED CARBON WIRE IN A SUBSEQUENT REACTOR AT A TEMPERATURE OF 700*-1400*C., AND EXPOSING THE WIRE IN SAID SUBSEQUENT REACTOR TO A GASEOUS STREAM OF A BORON HALIDE ADMIXED WITH HYDROGEN TO EFFECT DEPOSITION OF ELEMENTAL BORON THEREON; SAID DEPOSIT OF PYROLYTIC GRAPHITE BEING DISPOSED TO PREVENT FRACTURE OF THE CARBON WIRE BY THE BORON.

64-29 5 XQ REZ 8 v 3 l 2 5R 1975 M. BASCHE ETAL Re. 28,312

METHOD FOR PRODUCING HURON-CARBON FIBERS Original Filed March 27, 1969United States Patent 28,312 METHOD FOR PRODUCING HURON-CARBON FIBERSMalcolm Basche, West Hartford, Conn., Roy Fanti, Springfield, Massn,Francis 5. Galasso, Manchester, and Urban E. Kuntz, East Hartford,Conn., and Richard D. Schile, Hanover, NJL, assignors to United AircraftCorporation, East Hartford, Conn.

Original No. 3,679,475. dated July 25, 1972, Ser. No. 811,072, Mar. 27,1969. Application for reissue Oct. 5, 1972, Ser. No. 295,393

Int. Cl. [344d 1/14, 1/18; HOlb 1/00 US. Cl. 117-216 4 Claims Matterenclosed in heavy brackets appears in the original patent but forms nopart of this reissue specillcation; matter prlnted in italics indicatesthe additions made by reissue.

ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This inventionrelates generally to a method for producing boron fiber and moreparticularly relates to a method for continuously depositiong arelatively thick,

substantially constant diameter boron coating on a carbon filament.

It is known that filamentary boron may be produced by pyrolytictechniques in a process wherein the boron is chemically deposited on aresistively heated carbon monofilament which is exposed to a reactantgas consisting of boron trichloride admixed with hydrogen.

The use of carbon as a filamentary substrate for boron has beenrecognized as ofiering the potential of significant improvements in thefield of composite materials. Carbon, which in the present disclosurealso includes graphitic material, possesses desirable characteristics inthe form of electrical conductivity, hot strength, apparent chemicalcompatibility with boron, low density and an attractive cost feasibilityrelative to presently used tungsten filamentary substrates. Although thepotential of carbon as a substrate is thus recognized. realization ofthis potential has been limited by the degradation of the carbon fiberduring the coating process. It has been observed that, although thedeposition of boron on the carbon substrate can be initiated uniformly,the coating quickly takes on a hamboo-like appearance with periodicnodes of boron thickened circumfcrentially along the fiber. The areas ofincreased deposition are caused by the appearance of a plurality of hotspots along the fiber and subsequent tests have revealed that the hotspots are caused by fractures in the carbon core which produce anirreversible change in the electrical properties of the fiber. It wasfound that the fractures occur irrespective of whether the process bestatic or continuous and with the fiber at a uniform temperature.Further investigations have indicated that the substrate fracturing l5attributable to an unexpected growth phenomenon. As the boron isdeposited on the carbon it undergoes a period of expansion which. whenunchecked. exceeds the strength of the carbon filament. and causestracturing thereof. The exact cause and nature of this phenomenon isimperfectly understood at this time.

Recently, several techniques have been developed to improve theetlecuvencss of the basic continuous process through the close controlof process conditions. in one of these methods, a continuous coating ofnode-fr e :uuor phous boron is achieved by carefully limiting residentexposure of the carbon substrate in the reactor to a time period shorterthan that at which fracturing occurs At present however. the thicknessof node-free boron which can be deposited on a one mil carbon filamentby this technique is limited to a maximum of .6 mil to give a compositefiber of 2.2 mils.

SUMMARY OF THE INVENTION The present invention relates to the productionof relatively large, constant diameter composite fibers. approximating 4mils, in an improved process wherein filamentary carbon is modifiedthrough pretreatment. The lllVCllIlOll contemplates a process wherein anon-reactive structural barrier is provided between the carbon and theboron in the nature of an electrically conductive precoating olpyrolytic graphite. In one particular embodiment of the invention, thepyrolytic graphite is deposited in thin layers to provide for relativeslippage, without fracture, between the inner carbon bonded layer andthe outer boron bonded layer to prevent hot spotting and provide atechnique wherein relatively thick node-free boron coatings are achievedin reproducible fashion.

BRIEF DESCRIPTION OF THE DRAWING In the detailed description whichfollows. it will be convenient to make reference to the drawing whichshows. in cross sectional view, an elevation of a reactor usable in thepractice of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing, areactor 10 is shown and described below. It is to be understood,however. that although a single reactor is shown which is suitable forboth carbon and boron deposition, a plurality of such reactors arepreferably disposed. in sequence in the practice of the presentinvention. The reactor 10 comprises a tubular containment vessel 12having dual gas inlets I4. 16 at the upper end and dual gas outlets orexhaust ports 18, 20 at the lower end thereof. During the deposition ofpyrolytic graphite, the inlets l4 and 16 are utilized as a feed for areactant gas mixture comprising a diluent gas, as for example the inertgas argon, and a C3l'bOll-COl1lZlll'ling gas such as methane. During theboron deposition, the inlets l4 and 16 are utilized as the feed for areactant gas mixture comprising a boron halide and hydrogen. Thecontainment vessel is typically formed of quartz or Pyrex. although awide variety of other dielectrics and glasses are suitable. The gasinlet 16 and outlet 20 penetrate and are electrically connected to themetallic end plugs 24 and 26 which provide the end closures for thecontainment vessel and also. provide a convenient means through whichthe power may be supplied to the wire for resistance heating purposes.

The end plugs 24 and 26 are respectively formed to provide a well 30 and32 for containing a conductive sealant 34. such as mercury. The mercuryserves the dual purpose of p oviding a gas seal around the wire where itpenetrates the end plugs and further providing elec tricnl contactbetween the wire and the end plugs. tllrtiupt the gas tubes 20 and 26.the leads 23 and Z5. and the DC power source 36. The end plugs arefurther prmulcu with an annular surface groove 38. which communicateswith the mercury well 34 in the plugs through passage-- ways 40 and 42.to provide sealing between the plug and the abutting wall of thecontainment vessel where-u; gas

3 is prevented trom escaping from the reactor around the periphery orthe plutrs.

The respective plugs are further each formed with centrally orientedoritict-s. 44 and 46. which are large crtuttuit to permit free passageof the wire therethrough but which. in combination with the wire. aresmall enough to retain the mercury. through stll'fitCt? tension forces.in the re pective ttells. 'l he end plug can be modified to include anorilieed ruby. tungsten or other suitable insort through which the wirepasses and which provide the sealant retainment function previouslymentioned.

In the process of the present invention. a plurality of reactors [U areserially disposed and a filamentary substrare 50 is drawn therethroughfrom a feed reel 52 to a take up reel 54 which maintain the wire under aslight tension as it passes through the orifice openings. Power from DCsource 30 to the filament may be conveniently controlled by a resistor56 although other means are suitable in .arryinc at the process whereingraphite is deposited on the carbon substrate in the reactor, conditionsconventionally used for etlecting pyrolytic deposition of graphite maybe used. For example, the carbon filament substrate may be resistivelyheated to a temperature in the range of 1600 to 2100' C. preferably 1900to 2000 C. Temperatures above 1600 C. are needed to insure the formationt graphite rather than pure carbon. The reaction may be carried out at apressure of one atmosphere A reactant gas which is introduced into there actant chamber can be any carbon-containing gas suitable tordepositing pure carbon in graphitic form. In particular. methane. in anamount to 50 mol percent has given satisfactory results. The restrictedconcentration 01 methane in the reactant gas mixture is designed toprevent nodules troin forming from too high a concentratinn and inprevent the formation of soot. The reactant gas mixture also includes adiluent gas such as nitrogen. hydrogen or one of the inert gases. Argon,in an amount of St) to 9t) mol percent has been particularly metal Aprererrett ratio in the reactant gas mixture is mol percent methane and80 mol percent argon. Similarly, those conditions suitable for effectingpyrolyti'c boron deposition may also be used. For example. thegraph|te-coated carbon substrate may be resistively heated to atemperature in the range of 700 to 1400' C., preterahly 1100" to 1300"C. The reaction may be carried our .11 a pressure of one atmosphere andthe reactant gases may contain a boron containing gas leg. borontrichloride) in an amount of 15 to 75 mol percent and a reducing gas.preferably hydrogen. in an v amount 85 to mol percent. A preferred ratioof gases is 4o"? boron trichloride and b0 mol percent hydrogen.

During one investigation, a one mil carbon monofilamerit. trom GreatLakes Carbon Corporation. having a clean surface substantially free ofimperfection. a circular cross section and an electrical resistancebetween 500 to 25th) ohms per men. was coated with two layers oi. piolytic graphite prior to boron deposition. The mono tilament was passedthrough two reactors such as do scribed above. each having an etfectivelength of 3% inches In each reactor. the argon was provided at 800cwmiri and the (H at IOU cc./min. with a wire speed ot l U tt..='hr. Asubstantially constant current or' 145 ma gave a substrate tcmpeiatuieof 1900" C. The original diamete r t the lilamerii was 1.05 mil and.utter the first pylfi l' tpiitk' coating. the diameter Wt'LS 1.16 mils.Alter ir-t ritifld oyrogrtiphite coating. the diameter was L3 nuts Thepyrolytic graphite coated fiber as then passed hrough a boron reactor.The ichicwd with boron trichloiidc tccd of JUL) cc. min. and a hydrogenteed ot htltl (L/il'ltfl with .t tibcr speed ot tlt Hwhl through the ainch reactor The tibcr temperature was approximately 1200 C and thediameter measured $.11 mils The second boron layer as at:

tirsr boron layer as eomplihcd with a boron trichloride feed 0? 400CC./Iflll'l. and a hydrogen feed of 600 ce/min with a fiber speed of liOft./hr. The fiber temperature was 1170 during deposition and thediamctcr measured 2.9 mils. The third boron layer was achieved with aBC1 feed of 4nd cc./rnin and a hydrogen feed of 600 cc./m|ri with afiber speed 0t 1S0 ftn'hr. The fiber temperature during this pass wasmaintained a about i200 C. There were no hot spots and no breakage ofthe carbon monolilamerit and the diameter of the final compositefilament was smooth and uniform and measured 3.73 mils constant within100i]! inch By way or comparison, a boron-carbon fiber was producedwithout pyrographite precoatings on the carbon A one mil carbonmonofilament was run through the reactor at a speed of 230 fL/hr. and ata temperature of. H70 C with a BC]; feed or 400 cc./miri and an H; feedof 600 cc /min The resulting composite (X hibitcd the undesirable bamboostructure as previously discussed. The fiber had an average diameter ol3 mils with nodes as large as 4 mils in diameter which were spaced 7 toit) mils apart What has been set forth above is intended primarily asexemplary to enable those skilled in the art in the practice of theinvention and it should therefore be understood that, within the scopeof the appended claims, the invention may be practiced in other waysthan as specifically described.

What is claimed is: 1 A method for continuously depositing node-freeamorphour boron to uniform thicknesses greater than 0.6 mil on a movingresistively heated carbon wire as it is drawn through a series ofreactors comprising the steps of.

maintaining the carbon wire in the first reactor at a temperature of1600-2l00 C.;

exposing the carbon wire while at said temperature in at least onereactor to a gaseous stream consistin essentially of a carbon-containinggas admixed with a diluent gas to effect deposition of pyrolytic graphite on the carbon wire;

maintaining the graphite-coated carbon wire in a subsequent reactor at atemperature of 700-1400 C, and

exposing the wire in said subsequent reactor to a gaseous stream of a1307"! halide admixed with by drogen to effect deposition of elementalboron thereon;

said deposit of pyrolyttc graphite being disposed to prevent fracture ofthe carbon wire by the boron. Z The method of claim 1 wherein saidcarbon-com taining gas is methane and said diluent gas [5 argon. saidargon being present in an amount of 90 mol percent 3 The nvention ofclaim 1 wherein the pyrolvtic graphite coating is deposited in at leasttwo layers 4 A "Iflh ll! lo conrmuouslv dcportmig none-free u' IOTD IUuSboron to uniform thicknesses greater rtum 0.0 "it! on a mount;resistive! mited carbon titre as H U drown Trough a sertes 0f reactorscomp ising the ilepa mmatrrrurning "re carbon wire in the first reactorat a temperature rulficient to eflecr deposition 0/ pyr0- I\ :10grriphtre thereon, said temperature being abate 1601) (l erpnsi'i thecarbon wire while at .nri'd temperrirurr' in at least one reactor 10 i1gaseous Stream POILHXHIU; essentially of a carbon-containing gtirridmtxcd' ii H ri titlltt'll! gas to eflerr deposition of pyrolyrir.griipliirr on r te cu /101i wire;

mmrimiutu the .i,'mpltirt'-roiiri-tf carbon wire tn .1 subdream of aboron halide (tr/mun] wit/i livilro -t'ri 1c iii-u! tlt'PlMl/[Oll oft'it'mi'rtlul boron thereon. and

5 mid deposlr w pyw y xrapime being dis osed :0 3,567,826 preventfracture a! {he carbon WHL' by the boron. 3,369,920 3,464,843 ReferencesCited 3 79 2 5 following rcferenccs, cited bv the Examiner, are 5 ofrecord m the patented file of lhws patcnt or [he onginal pu enl UNITEDSTATES PATENTS 2,767,289 lO/I956 RObII'lSOH IN-D1610 3,226,248 12/1965Talley [IT-DIG. 10

2/1968 2/1968 9/1969 l l/l969 9/1970 2/l97l 6 Heestand at al. 11 -46 (7G Bourdeau 117-46 CG Baschc -7 ll7--4h (l G Morelock ll7--4h L1G Turkatll7 6 CG Morclock 117-1316v 10 MICHAEL SOFOCLEOUS, Primary Exammcr US.Cl. XR.

117 -46 (1.6.. 69, 93, 106 R, DIG, 10; 423- 4

1. A METHOD FOR CONTINUOUSLY DEPOSITING NODE-FREE AMORPHOUR BORON TO UNIFORM THICKNESSES GREATER THAN 0.6 MIL ON A MOVING RESISTIVELY HEATED CARBON WIRE AS IT IS DRAWN THROUGH A SERIES OF REACTORS COMPRISING THE STEPS OF: MAINTAINING THE CARBON WIRE IN THE FIRST REACTOR AT A TEMPERATURE OF 1600*-2100*C.; EXPOSING THE CARBON WIRE WHILE AT SAID TEMPERATURE IN AT LEAST ONE REACTOR TO A GASEOUS STREAM CONSISTING ESSENTIALLY OF A CARBON-CONTAINING GAS ADMIXED WITH A DILUENT GAS TO EFFECT DEPOSITION OF PYROLYTIC GRAPHITE ON THE CARBON WIRE; MAINTAINING THE GRAPHITE-COATED CARBON WIRE IN A SUBSEQUENT REACTOR AT A TEMPERATURE OF 700*-1400*C., AND EXPOSING THE WIRE IN SAID SUBSEQUENT REACTOR TO A GASEOUS STREAM OF A BORON HALIDE ADMIXED WITH HYDROGEN TO EFFECT DEPOSITION OF ELEMENTAL BORON THEREON; SAID DEPOSIT OF PYROLYTIC GRAPHITE BEING DISPOSED TO PREVENT FRACTURE OF THE CARBON WIRE BY THE BORON. 